Hypertonia is a condition of abnormally increased resistance to externally imposed movement about a joint. It may be caused by or manifested as spasticity, dystonia, rigidity, or a combination of such symptoms. Spasticity is a velocity-dependent resistance of a muscle to stretching. Therefore spasticity can be defined as hypertonia in which one or both of the following signs are present: (1) resistance to movement that increases with increasing speed of stretch and varies with the direction of joint movement, and (2) resistance to movement that rises rapidly above a threshold speed or joint angle. This form of hypertonia is common in patients with neurological conditions such as cerebral palsy, stroke, spinal cord injury, traumatic brain injury, or upper motor lesions. During clinical evaluation, spasticity manifests itself as a “catch” or increased resistance during the application of passive joint range of motion at or above a threshold velocity. The foregoing definitions are explained in greater detail in Sanger, T. D. et al., Classification and Definition of Disorders Causing Hypertonia in Childhood, Pediatrics 111:e89-e97 (2003).
Clinical classification systems have been designed in attempts to characterize the spastic reflex. Among the known classification systems are the Ashworth and the Tardieu systems. The Ashworth classification is based on the subjective perception of an examiner of the amount of increased resistance to movement felt during a passive range of motion maneuver of a patient's limb. The Tardieu system sought to be more objective and distinguishes specific angles of the limb joint as references for events during the performance of the passive range of motion maneuver. Thus, it identifies the spastic reflex by the angle at which resistance is first detected (R1) and the maximal extent of the angular range of motion (R2).
The Ashworth and Tardieu classification systems rely on examiner-dependent observations. Therefore, they are prone to subjectivity and variance from examiner to examiner and between examinations performed at different times.
Devices designed to objectively quantify muscle function are known. For example, a computer-assisted hand-held dynamometer (CAHNDY®) was developed to make dynamic measurements of muscle function. That device includes a strain gauge force transducer and an electrogoniometer. The former measures the amount of resistance applied to the limb and the latter measures the angle of rotation of the limb about an axis, typically a joint. The signals from the force transducer and the electrogoniometer are transmitted to a computer where they are processed to provide graphs of net moment on the limb as a function of the angle of displacement.
The MYOTONOMETER® device is a handheld electronic device that is designed to measure data for quantifying muscle stiffness. It does this by making a noninvasive assessment of muscle tone when the muscle is at rest and during contraction of the muscle. The probe measures muscle displacement under various conditions of applied force.
The BIODEX system is a manually operated therapeutic and diagnostic apparatus with an output monitor. It is usually used as an isokinetic machine to measure knee function and for knee rehabilitation. It has been used to measure the stretch reflex changes after intrathecal baclofen dosage adjustments in cerebral palsy patients.
Although the foregoing devices and systems provide useful information on muscle function and condition to the clinician, their utility in reliably and effectively quantifying hypertonia and presenting data characterizing such conditions is limited. More specifically, the known devices and systems and devices leave something to be desired with respect to the processing of the raw data and with respect to the presentation of the data in user-friendly graphical and absolute value formats. Accordingly, it would be desirable to have a device or system that is capable of reliably measuring indicia of hypertonia. It would also be desired that such a device can be used in a standardized examination protocol to provide real-time quantitative measurements of relevant parameters that are objective and reproducible.